1. Field of the Invention
The present invention relates to magnetic circuits with opposing permanent magnets and to methods for adjusting magnetic fields of such magnetic circuits.
2. Description of Related Art
For magnetic field generating devices using permanent magnets for magnetic resonance imaging apparatuses (also referred to MRI), the use of magnetic circuits with opposing permanent magnets utilizing rare earth magnets is well-established. As detailed below, such magnet opposing-type magnetic circuits include permanent magnets and a yoke for allowing a magnetic flux from the magnets to pass therethrough, and generally also include pole pieces that are preferably made of soft magnetic material such as soft iron, provided on surfaces of the permanent magnets in order to generate a uniform magnetic field in the gap between the permanent magnets. Usually, the magnetic field is adjusted by providing a holding plate of shim material, which is called a shim holder, on the pole piece, an inclined magnetic field coil or the like, and providing a shim member of such as the soft magnetic bodies or permanent magnet pieces thereon.
The clarity of MRI images is greatly affected by the uniformity of the magnetic field. Thus, when manufacturing MRI apparatuses, it is exceedingly important to arrange so as to easily obtain a uniform magnetic field.
Japanese Patent Provisional Publication No. H1-164356/1989 A, Japanese Patent Provisional Publication No. H9-56692/1997 A, Japanese Patent No. H5-87962/1993 B, all of which are herein incorporated by reference, and the like teach a method for adjusting a magnetic field using soft magnetic members or permanent magnets. For example, a method is shown in Japanese Patent Provisional Publication No. H1-164356/1989 A in which a plurality of small permanent magnet pieces for magnetic field adjustment are fastened by screws to a holding plate, which is arranged in proximity to the pole piece.
When using soft magnetic members for adjusting magnetic field, it is possible, for example by using thin plates of the soft magnetic members, to comparatively easily change the thickness and size thereof, and particularly it is also possible to prepare small members. On the other hand, it is more preferable to use permanent magnet pieces for adjusting the magnetic field than using the soft magnetic members because permanent magnet pieces can be used to adjustably weaken the magnetic field where necessary by using their polarity in a direction opposite to the principal magnetic field. However, up to now, the size of the magnet pieces, for example, that is actually used has not been specified, precisely what kind of magnet pieces should be used has not been clear, and fine adjustment of magnetic fields has been difficult.